Pulse-echo system utilizing an electron buncher tube



Nov. 21, 1950 J. ROTHSTEIN 2,530,401

PULSE-ECHO SYSTEM UTILIZING AN ELECTRON sumcuma TUBE Filed Jan. 14, 19433 Sheets-Sheet} 44 46 A Q ARE 4o r M i TOR\ PHASE SHIFTER 45 fi l'l'l ll l-l sauAcgEt/ any- 36 GENERATOR/ llllllll ll lllllllllll "ll Illlllllll| FIG. 7

INVENTOR JEROME ROTHSTEIN BY MW FIG.3

IATTOR EY J. ROTHSTEIN Nov. 21, 1950 PULSE-ECHO SYSTEM UTILIZING ANELECTRON BUNCHER TUBE Filed Jan. 14, 1943 3 Sheets-Sheet 2 INVENTORJEROME ROTHS EIN ATTORNEY J. ROTHSTEIN Nov. 21, 1950 PULSE-ECHO SYSTEMUTILIZING AN ELECTRON BUNCHER TUBE 3 Sheets-Sheet 3 Filed Jan.'14, 1943FIG. II

FIG. I2

INIENTOR JEROME ROTHSTEI N A 'I'TORN E Y system and their relation oneto" i'actured and used by or for the Governme H governmental purposes,without the paymen me of any royalty thereon.

This invention relates to apparatus for ing pulses. and for utilizingsame.

The primary object of my invention is,- I erally improve pulse echosystems, an mo it; especially apparatus for producing pulses i andotherpurposes. e v y A more particular object is to generateoi exceedinglyshort duration. so much'so they maybe fed directly to the radiating mehers, instead of being used merely to k y di frequency oscillator.

time in a limited region of space unoc other ponderable matter. Such adis bu charge will be termed quasi-stationary M may be collectedsubstantially instan n V 3 thus providing the desired short strongp Tothe accomplishment of the foi'fgo eral objects. andother morespeciflcpfiiec will hereinafter appear, my inventio in the pulser tubeelements and th are hereinafter more particularly (1 followingspecification. The speciil tio companied bydrawingsinwhich:

Figure 1 is a wiring diagram for crating tube embodying features otiori'f Figures 2 and 3 are respectively pl 1 elevations showing onemethod ofpr tube with a magnetic ileid necessa tion; 5

Figure 4 represents an alternating I, p 4 which may be used to controlthe p m me h e lslltmiihe tuheris shown? the tube: a t-imp anmatinmzcurrentwdiarinmtheg Figures 5 and 6 show square QCPRMmilgmyziszsuppiiedzbymsumbie:

n phthe relatively eeunmuiscs iigamthenedtnlmomme be derived from thesine wave otvliisureit and? applied .to the control and collecting hofthetube; ac? x51? n em'esor I Figure-lrepresentsthepuisesas-manifosted in i!!! .Y the antenna or other circuitutilizing 'the sami i m mi" Figure a is a section taken in elevatin thioz i fi a pulser tube'embodyins w w flf a n e. i sa ma h: 43m g is i whenI I .llgqlirmm'fiiilnichirmmf m iigg howuae1 number oi pulse:- tubes be53 t m :1 un by a I z or 8, beingconnectedto'thesquare Figure l0 is ablock diagram schematically wave generator I! by conductor 82. The otherconductor 34 from square wave generator 26 is connected to the cathode28. A suitable bias may be introduced, as is indicated by the battery36. With this arrangement the electron stream will be intermittent, itflowing during the positive half and being interrupted during thenegative half of the square wave. The resulting electron stream pulsesare of proper frequency but of longduration. The use of a square wave ismerely illustrative, other wave forms being suitable as described later.

Another part of the alternating current from source 24 is fed through aphase shiiter 38 and thence to another square wave generator I. Thephase shifter 38 may simply reverse the phase of the alternatingcurrent, in which case a simple inverter may be used, and the squarewave output from generator 40 will be 180 out of phase with the outputfrom generator 26. If desired the square wave output of generator 40 maybe rectilied to leave only the positive halves of the cycle,

its output being represented by Figure 6.

This output is fed to the collector plate It.

' In the present case the output is applied across a'resistor 42connected in'serieswith collector It. A radio frequency choke 44 may beput in the circuit to confine the radio frequency or pulse energy to theoutput lead 48. It will be evident that the operation is properlysynchronized, there being no collecting potential on collector l6 duringthe flow o! the electron stream and its formation into a ring, and therebeing no electron stream other than the formed ring when the ring isbeing collected by the potential then applied to the collector It. Theinstantaneous collection of the entire ring of electrons results in asingle sharp pulse, marked 48' in Figure 7. The additional oscillationsshown in Figure 'I take place when the pulse is fed to a radiator orother circuit tending to resonate. The pulse duration is so extremelyshort that in'practice the spacing between pulses will ordinarily bemuch greater, relative to the duration of the pulse, than is shown inFigure 7. Reverting now to Figure 8, the formation and collection of theelectron ring preferably takes placewithin an electrostatically shieldedspace, provided in the present case by a cylindrical shield 48. Thecylinder is made of a highly conductive metal and is preferably closedat the top and bottom, thereby providing a space which is shieldedagainst electrostatic fields but not against the'magnetic field. Anopening 50 is provided in the side wall of the shield near one end, andthe collector plate It is a disc disposed near the other end of theshield. The leadin 52 ofvcollector I6 is insulated from the shield bymeans of a glass bead II. The shield 48 is supported by a lead-in 56which also permits the application of a high positive potential to theshield (as isschematically indicated by the batteryit in Figure 1. Inpractice all necessary potentials may be taken from a suitable powersupply pack, rather than from batteries).

The electron gun It (Figure 8) may be dis posed horizontally and aimeddirectly into the opening SI, but for reasons discussed later, Iprefer-to dispose the gun vertically and to deilect the electron streamfrom vertical to horizontaldirection, as is indicated in Figure 8. Thenecessary deflection may be obtained by means of a curved deflectingplate 80 carried by a leadin 62, to which a suitable negative potentialmay be applied. Figure 1 shows a battery Blior It the electron gun weremounted horizontally at the side of the'tube, the deflecting electrodewith its attendant defocussing would be eliminated. However. in suchcase the gun design would be far more complicated, because the magneticfield would fringe over into the gun structure and would modify .themotion of the electrons even while they were being accelerated.Moreover. the focusing action 01 the magnetic field which obtains whenthe electrons are projected along the lines of force would be lost.

The electron gun itself may be of conventional character (see Figure 8),it comprises the usual heating filament and leads, the-usual concavecathode 28. and appropriate focussin electrodes 66 and 68, in additionto the intensity grid or control electrode 30. Appropriate lead-in wiresare provided for each of the electrodes, and these act also as supportsfor the same. The generally concentric relation between the tube and thesolenoid l8 surrounding the same is shown in Figures 2 and 3 of thedrawing.

A tube or the type described will yield a large power output. Fairlyheavy beam currents are obtainable from the electron gun. The-operatingvoltages may be made very high because the tube is large enough toinsure adequate insulation without introducing the transit timediiliculties encountered in conventional ultrahigh irequency circuitswhen dimensions are increased.

For a greater power output than that obtainable irom a single tube. aplurality of pulser tubes of the type here described may be used. Anumber of tubes may be operated in parallel, such an arrangement beingshown in Figure 9, in which each of the pulser tubes with associatedcircuits is indicated at 88,-these being connected to a plurality ofdipoles or other directive radiators 90 (only one of which is shown) bymeans of concentric or other suitable transmission lines 92. To ensuresynchronization all of the units are preferably energized from a commonalternating current source 24. All or the tubes may have common fieldcoil excitation, and a common power-supply circuit, including the squarewave generators and phase shifter. In addition, fine adjustment phasechanging means may be provided to synchronize the pulses despitevariations in tube parameters. If desired, the transmission lines 92 maybe made adjustable in length, thereby acting as the phase adjustingmeansfor the various antennas.

A complete pulse echo system using the present invention isschematically illustrated in Figure 10. In this figure the alternator 24corresponds to the source 24 in Figures 1 and 9. The transmitter 12comprises pulser tubes with a control wave generator and a collectorwave generator, all as was described in connection with Figures 1 and 9.The output of the transmitter is supplied directly to a suitableradiating system, schematically indicated at 14. While a single dipoleis shown. it will be understood that a directive array of dipoles isordinarily employed. The echo from the object intercepted by thetransmitted radio energy is picked up by a directive receiving antennaor array It. The echo energy is fed to a receiver 18, the output ofwhich is applied to the deflecting electrodes of an oscilloscope 80. Asweep wave chtained from a, sweep wave generator 84 is ap plied to thesweep electrodes of the oscilloscope 80. It will be noted that the sweepwave generator 84 is driven or synchronously controlledbyalternatimenergyircmtbeprimaryscureell."ltwlllbeunderstoodthatmoretbanoneosl'orexamp'leone arrays andreceivers." If we receiving may be used for each receiver, eitherphysically or electrically angle relative to one anand a suitable deviceused'io periodically one and then the other anadditional passages andinsulators (not shown) for the other conductors needed to energize andto bias the various electrodes in the tube. The

bushingsandinsulatorsforthispurposearedeto the'receiver." This'provldesa so-called dual receiver! for so-called "double tracking."

A wholly or partially blocking circuit 8! may be used to help thereceiver from the transmitter during radiation of the pulse.

Inasmuch ss'ule' echo is substantially a radio frequency 'peak. ratherthan a detected modulation offa radio frequency. carrier, the'durationof the echo pulse is exceedingly short. This will produces narrow peakon the oscilloscope screen. the trace may therefore be too faint at theTo merely increase the steady intensity of the electron is not desirableas it will help shorten the life of the cathode, and

will help burn out the fluorescent coating of the oscilloscope screen.Furthermore. increasing the sweep frequency so" as to allow the peak toreceive a larger fraction of the total illumination per-cycle isundesirable, for the sweep frequency is be synchronized with the pulse.rate. In accordance with a further feature of the present invention. thetrace is greatly intensifled solely during reception of the echo peak.

An arrangement-for this purpose is shown in 11. in which the sweepelectrodes are indicated at! while the deflecting electrodes areindicated at ill. The input signal from the reeeiver is applied to thedeflecting electrodes I I! by means of conductors Ill. The electron fromcathode "I. and its intensity by an intensity grid ill.

signed to withstand the necessary voltages. and are dimensioned toprovide sufllcient clearances to avoid corona discharge and the like.

As an aidin computing the proper. tube dimensions and pulse frequency, Imay point out that inasmuch as the electrons move in a planeperpendicular to the magneticfleld and are in an electrically held-freeregion. they will move ina circular path. The time taken for theelectron to make one circuit is given by the formula 2: Hc/m and theradius of the path is given by in which T is the time in seconds, H isthe magnetic field strength in Gauss. e/m is the ratio of the charge tothe mass of an electron in emu per gram. r'is the radius in centimeters,and p is the momentum of the electron in 665 units. The period of thecontrol wave if it is square (reciprocal of the pulse frequency) shouldbe The is focused by appropriate focusing electrodes m. 'rrne intensitygrid as is biased by' of a Ill. This bias is selected to normally keepthe cathode ray at a. rela- Y t ively low' or normal intensity level.The circult of the intensity grid III is also connected to the inputconductors I from the receiver. as

' by means of blocking condenser Ill and wire iil The echo peak whenapplied to the defleeting electrodesis a positive peak, and thispositive peak when fed to the intensity grid I" greatly magnifles theintensity of the cathode ray at the critical time when it must reproducethe peak. This feature is useful in any case where the signal to bepictured on the oscillo- Ml is of very short duration compared to theperiod of the sweep. and where for some reason it is undesirable to usea highsweep frequency. I use a large beam current momentarily.Preferably with a persistent screen. in order to get adequate visibilityfor the trace.

Inasmuch as ahlgh voltage is employed on the shielding and collectingelectrodes. the tube may if desired be shielded against X-rays. This isillustrated in Figure 12, in which the tube It!isenclosedwithin'aleadcasing I22.

Figure 12 also shows how the output of the tube may be connected to aconcentric line. The concentric line comprises a center conductor I24and an outer tube I". This is prefera ly enlarged at Ill to enclose thelead sheath I22.

less than or equal to but not greater than twice the needed to form onecomplete ring of electrons. In other words. the time may be limited sothat only a fraction of a ring'is formed for each' collection on thecollector, or the time may bein'creased until a complete ring is formed.

but any'greater increase is undesirable because the resulting mutualrepulsion of the electrons will spread the electrons. thus spoiling thedesiredinstantaneous collection which is essential for the productionofan exceedingly short pulse,

and what is even more important. causing some of the electrons to becollected by the shielding electrode 48.

The collection time is a function of the collecting voltage. thedistance between the plane of ring formation and the plane ofcollection. and the thickness of the electron beam. If the plane of thering is not perpendicular to the magnetic fleld, the ring will stretchaxially into a' ortion of a helix. and travel either toward thecollector or away from it during the formation time. Such an sheet willvitiate the operation. The extent of the harmfulness of this effect. inany particular case, is determlnedby the magnitude of the departure ofthe actual tube and beam geometry from the ideal. An additionalelectrode parallel to the collector and situated at the opposite end ofthe cylindrical shield will alleviate this condition. Ring .tilt" isthen overcome by the provision of small suitable D.-C. potentials to thetwo plates during the time of ring formation. Alternative methods are tocontrol the direction of the beam by varying the potential on thedeflecting electrode. thereby to ensure the desired perpendicularity, orto provide a local distortion of the magnetic field in the region ofring formation. This last may be accomplished by putting additionalturns on the soleof the field with rings of magnetic materia s.

The cathode is connected to the metal hous- Thedesiredneld-distortionmaybetreated much as is f done with field distributionsheretofore provided when fringing of a field was observed in focusingproblems encountered in the'operation of cyclotrons and inductionaccelerators. A third method is to replace the simple cylindrical shieldby a'more complicated one containing additional electrodes, barriers,projections, or extensions serving to give the desired electronic planarconcentration. Any residual tilt will' smear the pulse somewhat, thesmear being broader the larger the radius of the electron ring.

Withhigh collection voltages, small spacing, narrow beams and goodgeometry, pulses ofduration measured in very small fractions of amicrosecond may be obtained.

The use of a square control wave is not necessary. Departure from thiswave form simply yields a distribution in velocity of the ringelectrons, and thus a corresponding distribution in radii of curvatureof the trajectories. The circle is replaced by a spiral. A sawtoothwave, with alternate teeth blocked out, may be used. For acceleratingpotentials high enough to make the kinetic energy of the electrons anappreciable fraction of the rest ene y, e. g. of the order of 100 kv., anon-square wave will give a distribution in mass and thus a spread incollection time of the ring electrons. This relativistic pulsebroadening is usually negligible at all practical voltages. If agradually increasing or gradually decreasing voltage, for example,sawtooth, is employed, the period of formation may .be lengthenedrelative to the pulse period (the time T) because one then obtains amultiturn spiral rather than a ring. This gives a higher chargetransport per. pulse, and thus a larger power output. A disadvantage isthat lengthening the ring-forming time and increasing the number ofelectrons aggravates the defocusing effects of the mutual repulsions ofthe electrons, and so broadens the pulse. However, multiple turns arepossible with a spiral path, but should notbe permitted at all with acircular path.

It may be desired to vary the pulse frequency in order to vary themaximum range of the pulse echo system. The time T is fixed by the fieldstrength H, and the latter is preferably kept at a constant valuebecause of the inconvenience of changing the fieldstrength, with itsconcomitant change in radius of the electron ring. As indicated above,forming multiturn spirals enables one to use a lower pulse. frequencyfor a longer range without changing the field strength. The larger rangeis automatically accompanied by a larger power output, which is adesirable feature if the radio range is to keep up with thetheoretically available echo-time range.

If a higher pulse frequency (for shorter range work) than 1/ T isdesired, say n/T where n is an integer, and a full electron ring ratherthan i/nth of a ring is to be collected in order to maintain large poweroutput, 1; tubes may be employed instead of one. These tubes areconnected in parallel as in Figure 9, the only change being that each ofthe 1: tubes is made to differ from its neighbor in the phase of itsA.-C. excitation by Each tube has its own control andcollector wavegenerators. Those skilled in the art know how to obtain the desiredphase splitting. As an alternative, an n-phase alternating current maybe used, each phase feeding its own puller tube. For greater power, eachof the 1: tubes may be replaced by a number of tubes in parallel, asdescribed earlier in connection with Figure 9.

I prefer to dispense with shar tuning in .the receiver, and instead usean amplifier of. the linear type used for counting single particles innuclear physics. These amplifiers are of extremely high gain, can bestabilized-are of RC type, and have a finite time constant or resolvingtime, i. e. for incoming pulses closer together than a certain criticalamount, the output is a single large pulse. The composite transmittedpulse Should be completely keyed ofi in an interval smaller than theresolving time. This may be done by having sufiicient dipole damping. sothat the oscillation amplitude drops to substantially zero during theresolving time interval. The resolving time can always be increased byadjusting the RC values inthe receiver. The receiving dipoles arepreferably similar to the transmitting dipoles, this providingsuflicient receiver tuning for the present purpose.

The system has the advantage that the UHF is supplied without the use ofresonant circuits or conventional oscillators with their accompanyingdesign, adjustment and power output difliculties and limitations. Thepulse is the equivalent of a band of frequencies clustered about a meanfrequency given by j=1/t where t is the duration of thepulse-consequently length and spacing of the dipoles are not as criticalas is the'case where a conventional oscillator is employed. Transmittercircuits are considerably simplified. The keyer' is replaced by acontrol wave generator,.and amplifiers, oscillators, limiters, pulsesharpeners, etc. are eliminated. A rather large power output per tube isobtained because distributed and tube capacitance, transit time, etc. donot limit the size of the tube. Also, with proper design, high voltagescan be-usod and heavy beam-currentsare obtainable.- Plate dissipationcan be spread over a. plate of large area, and lasts for a period oftime much shorter than pulse durations currently employed, thuspermitting higher .power' output per tube per pulse. No dimculties withrespect to feed-back between output and input, parasitics, distributedcapacitance and inductance of leads, etc., prwent themselves.

It is believed that the construction and operation of my improved pulsertube and pulse echo system, as well as the many advantages thereof, willbe apparent from the foregoing detailed description. It will also beapparent that 7 while I have shown and described my invention in severalpreferred forms, many changes and modifications may be made withoutdeparting from the spirit of the invention as sought to be defined inthe following claims. In the claims the term "antenna is intended toinclude other directive radiators such as an electromagnetic horn, aswell as the use of reflectors, directors, lenses, etc.,' associated withradiating elements.

I claim:

1. In the operation of an electron tube comprising an electron sourceand an electron collector, the method of producing pulses which ineludesthe steps of forming and storing a mobile charge distribution during acertain period of time, causing said charge distribution to remainjuxtaposed to said collector at a fixed distance therefrom during saidperiod of time. and then collecting said mobile charge distribution inatime shorter than the formation time by suddenly assault creating an mchmto tie new in the renlentt teeneleetrenetrenme umenlaneeneeednemthecollector. shitting the phase oi another poralternating current byapteiydililf'deriving a collector wave from ti omgimat ofi 5 n r ;shii dalternating current wave, and anulaw wmcnuneiuaeemdweemmwi twWa r tfle'rgiute portion or the collector wave bmag'neti'c neld, a me"; to'j'aiggl'd" collector. whereby all of the elecdirection p dicular to "f q:the relatively long electron whereby taeeuetnrle m'dii e isllnje gnh g rnm at r pulses egathered almost simultaneously man el alternating'desired pulse frequency. th. 9

in l 1 .tc t'lle magneueneld 9 p. .tlllifiinaplanemba rrm mtsubstantially parallel to the collector. substantially shifting thephase of another portion of the aforesaid alternating current. derivinga collector wave from the phase-shifted alternating current. and 09 7 8the collector wave to the collector.

6. In the operation 0! an electron tube comprising an electron gun, acontrol grid. and an electron collector all surrounded by a solenoid,the method of generating extremely short pulses which includesgenerating an alternating current having the desired pulse frequency,deriving a controlwave from a portion oi the alternating current. soapplying the control wave to the control grid' of the electron gun thatintermittent long-duration pulses of electrons are dischargediromthegmguidingtheelectronstreamsina direction generally perpendicularto the magnetic fleidatapolntspacedi'romthecollectonsothat w de the iormrelatively powerful and grating tube comprising an elecco trolelectrode. a cylindrical eleccl d at both ends but having an ear oneend. a collector withthe opposite end, means to pply -a potential to thecontrol elecmake the electron stream interectrostatic shield having anopen- ;near one end, a collector within the opposite end, meansgeneratmagne' tic ileld within the shield, means to electron stream intothe opening in a U M la potential to the control r; ider to make theelectron stream tten means to periodically app y a ve to the collectorin order to colof each stream substanthe periodic potentials bet usbutoutot phase.

enerating tube comprising an c rol rid. a cylindrical electrostaticshield having an. opening in the side .of

one end, a collector disc withcylinder near the opposite end, means toelectron stream into the opening in a erse to the axis of the cylinder,a field in the direction or the aids or the cylinder, means to apply aperiodic potential to the control grid in order to make the electronstream intermittent. and means to apply a periodic positive potential tothe collector in order to collect all of the electrons of each streamsubstantially instantaneouslln. the n rlodic potentials beingsynchronous but out of phase.

, ll. Pulse generating apparatus comprising an electron source, acontrol electrode, a collector plate, a magnet providing a magneticfield, means to guide the electron stream in a direction perpendicularto the said iield, an alternating current generator having the desiredpulse irequency. a control wave generator driven thereby. connectionsapplying said control wave to the aforesaid control electrode. a phaseshifter for a portion oi the aforesaid alternating current, a collectorwave generator driven by the phaseto the magnetic field, means applyingthe resulting collector potential to'the collector.

12. Pulse generating apparatus comprising an electron source, a controlgrid for, said source, a cylindrical electrostatic shield having anopening in the side near one end, a collector near the opposite end,means to provide a magnetic field in said shield, means to guide theelectron stream into the opening in a direction perpendicular to thefield, an alternating current generator having the desired pulsefrequency, a control wave generator driven thereby, connections applyingsaid control wave to the aforesaid control electrode, a phase shifterfor a portion of the aforesaid alternating current, a collector wavegenerator driven by the phase-shifted alternating current, andconnections for applying the resulting potential to the collector.

13. Pulse generating apparatus comprising an electron gun, a controlgrid, a cylindrical electrostatic shield having an opening in the sidenear one end, a collector disc near the opposite end, means to guide theelectron stream into the opening in a direction parallel to thecollecting disc, a solenoid surrounding the tube with its axis in thedirection of the axis of the cylinder, an alternating current generatorhaving the desired pulse frequency, a square wave generator driven y.connections app y said square wave to the aforesaid control grid, 8-phase reverser for a portion of the aforesaid alternating current, asquare wave generator driven by the a pulse generator as defined inclaim '11 connected to said antenna, at least one directive receivingantenna, a tracking oscilloscope energized from said receiving antenna,and a sweep wave generator for the oscilloscope, said sweep wavegenerator being driven by a portion of the alternating current referredto in claim 11, whereby all parts of the system are maintained insynchronism.

15. A pulse echo system for locating and ranging, said system comprisinga directive antenna, a pulse generator as defined in claim 12 connectedto said antenna, at least one directive receiving antenna, a trackingoscilloscope enerin synchronism. Y

gized from said receiving antenna, and a sweep 16 A pulse echo systemfor locating and ranging, said system comprising a,directiv.e antenna, apulse generator as defined. in claim 11 connected to said antenna, .atleast one directive receiving antenna, a tracking oscilloscope havingdeflection electrodes energizedfrom said receiving antenna, a sweep wavegenerator for the sweep electrodes of said oscilloscope,,-s'aid sweepwave generator being synchronized bya portion of the alternating currentreferred to in claim 11, a control or intensity grid in saidoscilloscope, and means so connecting the receiving antenna to saidintensity grid as to increase the intensity of the trace duringreception of the echo.

17. A pulse echo system for locating and ranging, said system comprisinga directive antenna, an untuned pulse generator as defined in claim 11connected to said antenna, at least one directive receiving antenna, andan untuned receiver connected to said antennas.

18. A pulse echo system for locating and ranging, said system comprisinga directive antenna, an untuned pulse generator as defined in claim 11connected to said antenna, at least one directive receiving antenna, andan untuned receiver connected to said antenna, a tracking oscilloscopeenergized from said untuned receiver, and a sweep wave generator for theoscilloscope, said sweep wave generator being driven by a portion of thealternating current referred to in claim 11 in order to maintain allparts of the system in synchronism.

JEROME RUIHSTEIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS- Number Name 7 Date 1 2,032,620 Langmuir Mar. 3,1936 2,037,977 Hansell Apr. 21, 1936 2,143,035 Smith Jan. 10, 19392,218,549 Philpott Oct. 22, 1940 2,227,598 Lymanet al. Jan. 7, 19412,289,319 Strobel July 7, 1942 2,362,209 Litton Nov. 7, 1944 2,372,210Labin Mar, 27, 1945

